div (øu) = u · grad o + o div u

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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Let \( \phi = \phi(x) \), \( \mathbf{u} = \mathbf{u}(x) \), and \( \mathbf{T} = \mathbf{T}(x) \) be differentiable scalar, vector, and tensor fields, where \( \mathbf{x} \) is the position vector. Show that
Transcribed Image Text:Let \( \phi = \phi(x) \), \( \mathbf{u} = \mathbf{u}(x) \), and \( \mathbf{T} = \mathbf{T}(x) \) be differentiable scalar, vector, and tensor fields, where \( \mathbf{x} \) is the position vector. Show that
The image contains the formula for the divergence of a product of a scalar field \( \phi \) and a vector field \( \mathbf{u} \):

\[ \text{div} (\phi \mathbf{u}) = \mathbf{u} \cdot \nabla \phi + \phi \, \text{div} \, \mathbf{u} \]

**Explanation:**

- **\(\text{div} (\phi \mathbf{u})\)**: Represents the divergence of the product of \( \phi \) (a scalar field) and \( \mathbf{u} \) (a vector field).

- **\(\mathbf{u} \cdot \nabla \phi\)**: Represents the dot product of the vector field \( \mathbf{u} \) and the gradient of the scalar field \( \phi \).

- **\(\phi \, \text{div} \, \mathbf{u}\)**: Represents the product of the scalar field \( \phi \) and the divergence of the vector field \( \mathbf{u} \).

This identity is useful in vector calculus when dealing with fluid dynamics, electromagnetism, and other fields involving differential equations.
Transcribed Image Text:The image contains the formula for the divergence of a product of a scalar field \( \phi \) and a vector field \( \mathbf{u} \): \[ \text{div} (\phi \mathbf{u}) = \mathbf{u} \cdot \nabla \phi + \phi \, \text{div} \, \mathbf{u} \] **Explanation:** - **\(\text{div} (\phi \mathbf{u})\)**: Represents the divergence of the product of \( \phi \) (a scalar field) and \( \mathbf{u} \) (a vector field). - **\(\mathbf{u} \cdot \nabla \phi\)**: Represents the dot product of the vector field \( \mathbf{u} \) and the gradient of the scalar field \( \phi \). - **\(\phi \, \text{div} \, \mathbf{u}\)**: Represents the product of the scalar field \( \phi \) and the divergence of the vector field \( \mathbf{u} \). This identity is useful in vector calculus when dealing with fluid dynamics, electromagnetism, and other fields involving differential equations.
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